Process for the production of furfural

ABSTRACT

Furfural is produced by contacting a feedstock solution containing C 5  sugar and/or C 6  sugar with a solid acid catalyst using reactive distillation. Both high yield and high conversion are obtained, without production of insoluble char in the reaction vessel. Degradation of furfural is minimized by its low residence time in contact with the solid acid catalyst. Higher catalyst lifetime can be achieved because the catalyst is continually washed with the refluxing aqueous solution and not sitting in high-boiling byproducts like humins, which are known to be deleterious to catalyst lifetime.

This application claims the benefit under 35 U.S.C. §119(e) of U.S.Provisional Application No. 61/580,717, filed Dec. 28, 2011, which isherein incorporated by reference.

FIELD OF THE INVENTION

A method for the production of furfural and related compounds from sugarstreams is provided.

BACKGROUND OF THE INVENTION

Furfural and related compounds, such as hydroxymethylfurfural (HMF), areuseful precursors and starting materials for industrial chemicals foruse as pharmaceuticals, herbicides, stabilizers, and polymers. Thecurrent furfural manufacturing process utilizes biomass such as corn coband sugar cane bagasse as a raw material feed stock for obtaining xyloseor hemicellulose.

The hemicellulose is hydrolyzed under acidic conditions to its monomersugars, such as glucose, fructose, xylose, mannose, galactose, rhamnose,and arabinose. Xylose, which is a pentose (i.e., a “C₅ sugar”) is thesugar present in the largest amount. In a similar aqueous acidicenvironment, the C₅ sugars are subsequently dehydrated and cyclized tofurfural.

A major difficulty with known methods for dehydration of sugars is theformation of undesirable resinous material that not only leads to yieldloss but also leads to fouling of exposed reactor surface and negativelyimpacts heat transfer characteristics. Further, the use of solid acidcatalyst could also lead to coking issues.

A review by R. Karinen et al. (ChemSusChem 4 (2011), pp. 1002-1016)includes several commonly used methods of producing furfural generallyas described above. All of those methods involve use of a solubleinorganic acid catalyst, such as sulfuric, phosphoric, or hydrochloricacid. These acids are difficult to separate from the reaction medium orproduct stream. Low yields can result from formation of undesirablebyproducts. Further, their use can require increased capital costsbecause of associated corrosion and environmental emission issues.

There remains a need for a process to produce furfural and relatedcompounds from sugars at both high yield and high conversion.

SUMMARY OF THE INVENTION

In an aspect of the invention, there is a process comprising:

-   -   (a) providing a reactive distillation column comprising a top, a        bottom, a reaction zone in between the top and the bottom, and a        solid acid catalyst disposed in the reaction zone;    -   (b) bringing a feedstock solution into contact with the solid        acid catalyst for a residence time sufficient to produce a        mixture of water and furfural, wherein the feedstock solution        comprises C₅ sugar, C₆ sugar or a mixture thereof, and the        reaction zone is at a temperature in the range of 90-250° C. and        a pressure in the range of 0.1-3.87 MPa;    -   (c) removing the mixture of water and furfural from the top of        the reactive distillation column; and    -   (d) collecting water, unreacted sugars and nonvolatile        byproducts from the bottom of the reactive distillation column.

In an aspect, the process further comprises feeding a water-miscibleorganic solvent to the reaction zone.

In another aspect, the feedstock solution further comprises awater-miscible organic solvent.

In another aspect, there is a process comprising the steps of:

(a) providing a reactor comprising a reactive distillation columncomprising an upper, rectifying section; a lower, stripping section; anda reboiler, wherein the stripping section or the reboiler is a reactionzone containing a solid acid catalyst;

(b) continuously feeding a solution comprising C₅ sugar, C₆ sugar or amixture thereof to the column at a location between the rectifyingsection and the stripping section, allowing the solution to flow intothe reaction zone into contact with the solid acid catalyst, therebyforming a reaction mixture, wherein

-   -   (i) the temperature of the reaction mixture is between about        90° C. and about 250° C.    -   (ii) the reaction mixture is held at a pressure between about        atmospheric pressure and about 3.87×10⁶ Pa, and    -   (iii) the sugar solution and catalyst are in contact for a time        sufficient to produce water and furfural;

(c) drawing off a mixture of furfural and water at the top of thecolumn;

(d) collecting water, unreacted sugars, and nonvolatile byproducts inthe reboiler;

(e) removing nonvolatile byproducts from the reboiler; and

(f) removing the water and unreacted sugars from the reboiler forfurther use or disposal.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features and/or embodiments of this invention are illustrated indrawings as described below. These features and/or embodiments arerepresentative only, and the selection of these features and/orembodiments for inclusion in the drawings should not be interpreted asan indication that subject matter not included in the drawings is notsuitable for practicing the invention, or that subject matter notincluded in the drawings is excluded from the scope of the appendedclaims and equivalents thereof.

FIG. 1 is a schematic illustration of an exemplary reactor configurationused in the production of furfural in accordance with variousembodiments of the present invention.

DETAILED DESCRIPTION Definitions

As used herein, the term “sugar” includes monosaccharides,disaccharides, and oligosaccharides. Monosaccharides, or “simplesugars,” are aldehyde or ketone derivatives of straight-chainpolyhydroxy alcohols containing at least three carbon atoms. A pentoseis a monosaccharide having five carbon atoms; some examples are xylose,arabinose, lyxose and ribose. A hexose is a monosaccharide having sixcarbon atoms; some examples are glucose and fructose. Disaccharidemolecules (e.g., sucrose, lactose, fructose, and maltose) consist of twocovalently linked monosaccharide units. As used herein,“oligosaccharide” molecules consist of about 3 to about 20 covalentlylinked monosaccharide units.

As used herein, the term “C_(n) sugar” includes monosaccharides having ncarbon atoms; disaccharides comprising monosaccharide units having ncarbon atoms; and oligosaccharides comprising monosaccharide unitshaving n carbon atoms. Thus, “C₅ sugar” includes pentoses, disaccharidescomprising pentose units, and oligosaccharides comprising pentose units.

As used herein, the term “hemicellulose” refers to a polymer comprisingC₅ and C₆ monosaccharide units. Hemicellulose consists of short, highlybranched chains of sugars. In contrast to cellulose, which is a polymerof only glucose, a hemicellulose is a polymer of five different sugars.It contains five-carbon sugars (usually D-xylose and L-arabinose) andsix-carbon sugars (D-galactose, D-glucose, and D-mannose, fructose).Hemicellulose can also contain uronic acid, sugars in which the terminalcarbon's hydroxyl group has been oxidized to a carboxylic acid, such as,D-glucuronic acid, 4-O-methyl-D-glucuronic acid, and D-galacturonicacid. The sugars are partially acetylated. Typically the acetyl contentis 2 to 3% by weight of the total weight of hemicellulose. Xylose istypically the sugar monomer present in hemicellulose in the largestamount.

As used herein, the term “solid acid catalyst” refers to any solidmaterial containing Brönsted and/or Lewis acid sites, and which issubstantially undissolved by the reaction medium under ambientconditions.

As used herein, the term “nonvolatile byproduct” denotes a reactionbyproduct that either has a boiling point at one atmospheric pressuregreater than the boiling point of the distilled product(s), or is anonvolatile solid.

As used herein, the term “heteropolyacid” denotes an oxygen-containingacid with P, As, Si, or B as a central atom which is connected viaoxygen bridges to W, Mo or V. Some examples are phosphotungstic acid,molybdophosphoric acid.

As used herein, the term “high boiling” denotes a solvent having aboiling point above about 100° C. at one atmosphere.

As used herein the term “water-miscible organic solvent” refers to anorganic solvent that can form a monophasic solution with water at thetemperature at which the reaction is carried out.

As used herein the term “humin(s)” refers to dark, amorphousbyproduct(s) resulting from acid induced sugar and furfural degradation.

As used herein, the term “selectivity” refers to the moles of furfuralproduced, divided by the moles of xylose transformed to products over aparticular time period.

In an embodiment, there is a process for the production of furfuralcomprising providing a reactive distillation column comprising a top, abottom, a reaction zone in between the top and the bottom, and a solidacid catalyst disposed in the reaction zone. FIG. 1 shows a schematicillustration of an exemplary reactor configuration comprising a reactivedistillation column 10 comprising a top 11, a bottom 12, a reaction zone20 in between the top 11 and the bottom 12, and a solid acid catalyst 2disposed in the reaction zone 20.

The solid acid catalyst is a solid acid having the thermal stabilityrequired to survive reaction conditions. The solid acid catalyst may besupported on at least one catalyst support. Examples of suitable solidacids include without limitation the following categories: 1)heterogeneous heteropolyacids (HPAs) and their salts, 2) natural orsynthetic clay minerals, such as those containing alumina and/or silica(including zeolites), 3) cation exchange resins, 4) metal oxides, 5)mixed metal oxides, 6) metal salts such as metal sulfides, metalsulfates, metal sulfonates, metal nitrates, metal phosphates, metalphosphonates, metal molybdates, metal tungstates, metal borates, and 7)combinations of any members of any of these categories. The metalcomponents of categories 4 to 6 may be selected from elements fromGroups 1 through 12 of the Periodic Table of the Elements, as well asaluminum, chromium, tin, titanium, and zirconium. Examples include,without limitation, sulfated zirconia and sulfated titania.

Suitable HPAs include compounds of the general formula X_(a)M_(b)O_(c)^(q−), where X is a heteroatom such as phosphorus, silicon, boron,aluminum, germanium, titanium, zirconium, cerium, cobalt or chromium, Mis at least one transition metal such as tungsten, molybdenum, niobium,vanadium, or tantalum, and q, a, b, and c are individually selectedwhole numbers or fractions thereof. Nonlimiting examples of salts ofHPAs are lithium, sodium, potassium, cesium, magnesium, barium, copper,gold and gallium, and onium salts such as ammonia. Methods for preparingHPAs are well known in the art and are described, for example, in G. J.Hutchings, C. P. Nicolaides and M. S. Scurrel, Catal Today (1994) p 23;selected HPAs are also available commercially, for example, throughSigma-Aldrich Corp. (St. Louis, Mo.). Examples of HPAs suitable for thedisclosed process include, but are not limited to, tungstosilicic acid(H₄[SiW₁₂O₄₀].xH₂O), tungstophosphoric acid (H₃[PW₁₂O₄₀].xH₂O),molybdophosphoric acid (H₃[PMo₁₂O₄₀].xH₂O), molybdosilicic acid(H₄[SiMo₁₂O₄₀].xH₂O), vanadotungstosilicic acid(H_(4+n)[SiV_(n)W_(12-n)O₄₀].xH₂O), vanadotungstophosphoric acid(H_(3+n)[PV_(n)W_(12-n)O₄₀].xH₂O), vanadomolybdophosphoric acid(H_(3+n)[PV_(n)Mo_(12-n)O₄₀].xH₂O), vanadomolybdosilicic acid(H_(4+n)[SiV_(n)Mo_(12-n)O₄₀].xH₂O), molybdotungstosilicic acid(H₄[SiMo_(n)W_(12-n)O₄₀].xH₂O), molybdotungstophosphoric acid(H₃[PMo_(n)W_(12-n)O₄₀].xH₂O), wherein n in the formulas is an integerfrom 1 to 11 and x is an integer of 1 or more.

Natural clay minerals are well known in the art and include, withoutlimitation, kaolinite, bentonite, attapulgite, montmorillonite andzeolites.

In an embodiment, the solid acid catalyst is a cation exchange resinthat is a sulfonic-acid-functionalized polymer. Suitable cation exchangeresins include, but are not limited to the following:styrene-divinylbenzene copolymer-based strong cation exchange resinssuch as Amberlyst™ and Dowex® available from Dow Chemicals (Midland,Mich.) (for example, Dowex® Monosphere M-31, Amberlyst™ 15, Amberlite™120); CG resins available from Resintech, Inc. (West Berlin, N.J.);Lewatit resins such as MonoPlus™ S 100H available from Sybron ChemicalsInc. (Birmingham, N.J.); fluorinated sulfonic acid polymers (these acidsare partially or totally fluorinated hydrocarbon polymers containingpendant sulfonic acid groups, which may be partially or totallyconverted to the salt form) such as Nalion® perfluorinated sulfonic acidpolymer, Nafion® Super Acid Catalyst (a bead-form strongly acidic resinwhich is a copolymer of tetrafluoroethylene andperfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride, converted toeither the proton (H⁺), or the metal salt form) available from DuPontCompany (Wilmington, Del.).

In an embodiment, the solid acid catalyst is a supported acid catalyst.The support for the solid acid catalyst can be any solid substance thatis inert under the reaction conditions including, but not limited to,oxides such as silica, alumina, titania, sulfated titania, and compoundsthereof and combinations thereof; barium sulfate; calcium carbonate;zirconia; carbons, particularly acid washed carbon; and combinationsthereof. Acid washed carbon is a carbon that has been washed with anacid, such as nitric acid, sulfuric acid or acetic acid, to removeimpurities. The support can be in the form of powder, granules, pellets,or the like. The supported acid catalyst can be prepared by depositingthe acid catalyst on the support by any number of methods well known tothose skilled in the art of catalysis, such as spraying, soaking orphysical mixing, followed by drying, calcination, and if necessary,activation through methods such as reduction or oxidation. The loadingof the at least one acid catalyst on the at least one support is is inthe range of 0.1-20 weight based on the combined weights of the at leastone acid catalyst and the at least one support. Certain acid catalystsperform better at low loadings such as 0.1-5%, whereas other acidcatalysts are more likely to be useful at higher loadings such as10-20%. In an embodiment, the acid catalyst is an unsupported catalysthaving 100% acid catalyst with no support such as, pure zeolites andacidic ion exchange resins.

Examples of supported solid acid catalysts include, but are not limitedto, phosphoric acid on silica, Nafion® perfluorinated sulfonic acidpolymer on silica, HPAs on silica, sulfated zirconia, and sulfatedtitania. In the case of Nafion® on silica, a loading of 12.5% is typicalof commercial examples.

In another embodiment, the solid acid catalyst comprises Amberlyst™ 70.

In one embodiment, the solid acid catalyst comprises a Nafion® supportedon silica (SiO₂).

In one embodiment, the solid acid catalyst comprises natural orsynthetic clay minerals, such as those containing alumina and/or silica(including zeolites).

Zeolites suitable for use herein can be generally represented by thefollowing formula M_(2/n)O.Al₂O₃.xSiO₂.yH₂O wherein M is a cation ofvalence n, x is greater than or equal to about 2, and y is a numberdetermined by the porosity and the hydration state of the zeolite,generally from about 2 to about 8. In naturally occurring zeolites, M isprincipally represented by Na, Ca, K, Mg and Ba in proportions usuallyreflecting their approximate geochemical abundance. The cations M areloosely bound to the structure and can frequently be completely orpartially replaced with other cations by conventional ion exchange.

The zeolite framework structure has corner-linked tetrahedra with Al orSi atoms at centers of the tetrahedra and oxygen atoms at the corners.Such tetrahedra are combined in a well-defined repeating structurecomprising various combinations of 4-, 6-, 8-, 10-, and 12-memberedrings. The resulting framework structure is a pore network of regularchannels and cages that is useful for separation. Pore dimensions aredetermined by the geometry of the aluminosilicate tetrahedra forming thezeolite channels or cages, with nominal openings of about 0.26 nm for6-member rings, about 0.40 nm for 8-member rings, about 0.55 nm for10-member rings, and about 0.74 nm for 12-member rings (these numbersassume the ionic radii for oxygen). Zeolites with the largest pores,being 8-member rings, 10-member rings, and 12-member rings, arefrequently considered small, medium and large pore zeolites,respectively.

In a zeolite, the term “silicon to aluminum ratio” or, equivalently,“Si/Al ratio” means the ratio of silicon atoms to aluminum atoms. Poredimensions are critical to the performance of these materials incatalytic and separation applications, since this characteristicdetermines whether molecules of certain size can enter and exit thezeolite framework.

In practice, it has been observed that very slight decreases in ringdimensions can effectively hinder or block movement of particularmolecular species through the zeolite structure. The effective poredimensions that control access to the interior of the zeolites aredetermined not only by the geometric dimensions of the tetrahedraforming the pore opening, but also by the presence or absence of ions inor near the pore. For example, in the case of zeolite type A, access canbe restricted by monovalent ions, such as Na⁺ or K⁺, which are situatedin or near 8-member ring openings as well as 6-member ring openings.Access can be enhanced by divalent ions, such as Ca²⁺, which aresituated only in or near 6-member ring openings. Thus, the potassium andsodium salts of zeolite A exhibit effective pore openings of about 0.3nm and about 0.4 nm respectively, whereas the calcium salt of zeolite Ahas an effective pore opening of about 0.5 nm.

The presence or absence of ions in or near the pores, channels and/orcages can also significantly modify the accessible pore volume of thezeolite for sorbing materials. Representative examples of zeolites are(i) small pore zeolites such as NaA (LTA), CaA (LTA), Erionite (ERI),Rho (RHO), ZK-5 (KFI) and chabazite (CHA); (ii) medium pore zeolitessuch as ZSM-5 (MFI), ZSM-11 (MEL), ZSM -22 (TON), and ZSM-48 (*MRE); and(iii) large pore zeolites such as zeolite beta (BEA), faujasite (FAU),mordenite (MOR), zeolite L (LTL), NaX (FAU), NaY (FAU), DA-Y (FAU) andCaY (FAU). The letters in parentheses give the framework structure typeof the zeolite. Definitions of zeolite framework types may be found inthe following references: http://www.iza-structure.org/, and Baerlocher,McCusker, Olson[“Atlas of Zeolite Framework Types, 6^(th) revisededition, Elsevier, Amsterdam].

Zeolites suitable for use herein include medium or large pore, acidic,hydrophobic zeolites, including without limitation ZSM-5, faujasites,beta, mordenite zeolites or mixtures thereof, having a high silicon toaluminum ratio, such as in the range of 5:1 to 400:1 or 5:1 to 200:1.Medium pore zeolites have a framework structure consisting of10-membered rings with a pore size of about 0.5-0.6 nm. Large porezeolites have a framework structure consisting of 12-membered rings witha pore size of about 0.65 to about 0.75 nm. Hydrophobic zeolitesgenerally have Si/Al ratios greater than or equal to about 5, and thehydrophobicity generally increases with increasing Si/Al ratios. Othersuitable zeolites include without limitation acidic large pore zeolitessuch as H—Y with Si/Al in the range of about 2.25 to 5.

Zeolites with a high Si/Al ratio can be prepared synthetically, or bymodification of high alumina containing zeolites using methods known inthe art. These methods include without limitation treatment with SiCl₄or (NH₄)₂SiF₆ to replace Al with Si, as well as treatment with steamfollowed by acid. A SiCl₄ treatment is described by Blatter [J. Chem.Ed. 67 (1990) 519]. A (NH₄)₂SiF₆ treatment is described in U.S. Pat. No.4,503,023. These treatments are generally very effective at increasingthe Si/Al ratio for zeolites such as zeolites Y and mordenite.

The presence of aluminum atoms in the frameworks results in hydrophilicsites. On removal of these framework aluminum atoms, water adsorption isseen to decrease and the material becomes more hydrophobic and generallymore organophilic. Hydrophobicity in zeolites is further discussed byChen [J. Phys. Chem. 80 (1976) 60]. Generally, high Si/Al containingzeolites exhibit higher thermal and acid stability. Acid forms ofzeolites can be prepared by a variety of techniques including ammoniumexchange followed by calcination or by direct exchange of alkali ionsfor protons using mineral acids or ion exchangers. Acid sites inzeolites are further discussed in Dwyer, “Zeolite, Structure,Composition and Catalysis” in Chemistry and Industry, Apr. 2, 1984.

Certain types of molecular sieves, of which zeolites are a sub-type, mayalso be used as the catalytic material in the processes hereof. Whilezeolites are aluminosilicates, molecular sieves contain other elementsin place of aluminum and silicon, but have analogous structures. Largepore, hydrophobic molecular sieves that have similar properties to thepreferred zeolites described above are suitable for use herein. Examplesof such molecular sieves include without limitation Ti-Beta, B-Beta, andGa-Beta silicates. Molecular sieves are further discussed in Szostak,Molecular Sieves Principles of Synthesis and Identification, (VanNostrand Reinhold, NY, 1989).

Referring back to the process for the production of furfural, theprocess also comprises, as shown in FIG. 1 bringing a feedstock solution1 into contact with the solid acid catalyst 2 for a residence timesufficient to produce a mixture 5 of water 7 and furfural 8 in thereaction zone 20. In an embodiment, the feedstock solution 1 comprisesC₅ sugar, C₆ sugar or a mixture thereof dissolved in water, or a highboiling water-miscible organic solvent, or a mixture thereof. In anotherembodiment, the reaction zone is at a temperature in the range of90-250° C. and a pressure in the range of 0.1-3.87 MPa.

The feedstock solution comprises at least one C₅ sugar, at least one C₆sugar, or a mixture of at least one C₅ sugar and at least one C₆ sugar.Examples of suitable C₅ sugars, pentoses include without limitationxylose, arabinose, lyxose and ribose. Examples of suitable C₆ sugars,hexoses include without limitation glucose, fructose, mannose, andgalactose.

In one embodiment, the feedstock solution comprises xylose. In anotherembodiment, the feedstock solution comprises glucose. In anotherembodiment, the feedstock solution comprises comprises xylose andglucose.

The total sugar (C₅ sugar, C₆ sugar, or a mixture thereof) is present inthe feedstock solution in the range of 1-99 weight % or 0.1-50 weight %or 5-35 weight % or 5-10 weight %, based on the total weight of thefeedstock solution. In an embodiment, the feedstock solution 1 is anaqueous feedstock solution.

As shown in the FIG. 1, the feedstock solution 1 is added to thedistillation column 10 at a location between the rectifying section 16and the reaction zone 20 at a rate that provides sufficient residencetime in the reaction zone 20 (which is also the stripping section) forcomplete or nearly complete conversion of sugars to furfural. Therequired residence time is a function of temperature and sugarconcentration and is readily determined by one of skill in the art. Inan embodiment, the residence time in the reaction zone is in the rangeof 1-500 min or 1-250 min or 5-120 min. The feedstock solution 1 flowsdown through the reaction zone 20 and is converted to a mixture 5 offurfural 8 and water 7 which is then partially vaporized and refluxes aspart of the distillation column 10.

The temperature of the feedstock solution in the reaction zone 20 is inthe range of 90-250° C. or 140-220° C. or 155-200° C.

The reaction is carried at a pressure between about atmospheric pressureand 3.87 MPa or 0.1-3.4 MPa or 0.1-2.0 MPa. In an embodiment, thefeedstock solution is an aqueous feedstock solution and the reaction iscarried at a pressure in the range of 0.5-1.6 MPa. In anotherembodiment, the feedstock solution comprises a high boilingwater-miscible organic solvent, and the reaction is carried at aboutatmospheric pressure.

The process for the production of furfural further comprises removingthe mixture 5 of water and furfural from the top 11 of the reactivedistillation column 10 and collecting water and/or solvent unreactedsugars and nonvolatile byproducts into the reboiler 3 from the bottom ofthe reactive distillation column 10, as shown in FIG. 1.

As the reaction proceeds, a mixture 5 of vapors comprising one or moreof furfural, water, acetic acid, acetone, and formic acid are removedfrom the reaction mixture via reflux through a multistage distillationcolumn 10, condensed, and collected as a solution 5 of furfural andwater. The use of staging in the distillation process allows moreefficient stripping of furfural away from the acid catalyst solution.This increases furfural yield by driving the reaction toward completionand by minimizing formation of byproducts.

The sugar in the feedstock solution undergoes chemical transformation tofurfural, which, along with water (from the aqueous feedstock and waterproduced by the reaction), is then drawn at the top 11 of thedistillation column 10. This minimizes the residence time of furfural inthe acidic environment of the reaction zone 20 and thereby minimizes itsdegradation. The furfural 8 is separated from the water and purified byany convenient methods known in the art, and the product furfural isremoved. The water is either recycled to the source of the feedstocksugar solution or is released from the process.

Reaction byproducts 3, including, but not limited to, water, unreactedsugars, and non-volatile byproducts such as humins are collected in thereboiler 15 beneath the distillation column 10, as shown in FIG. 1. Thenonvolatile byproducts 4 are removed from the reboiler 15 (e.g., byfiltration). The solution 6 of water and unreacted sugars can bedisposed of, or at least a portion can be concentrated by evaporationand fed as a stream 6′ to be used as feedstock solution 1, as shown inFIG. 1.

In one embodiment, with reference to FIG. 1, the feedstock solution 1 isfed into the distillation column 10 at a location between the rectifyingsection 16 of the distillation column 10 and the reaction zone 20, abovethe solid catalyst 2. The catalyst 2 is included in the bottom,stripping section, which is the reaction zone 20. A mixture 5 offurfural and water (as steam) are drawn off at the top 11 of the column5. Reaction byproducts 3 such as, water and/or solvent, unreactedsugars, and nonvolatile byproducts (e.g., humins and other higherboiling byproducts) are collected in the reboiler 15. The nonvolatilematerials 4 are removed from the reboiler 15. The remaining solution 6is concentrated by evaporation, with evaporated water vapor removed fordisposal or reuse. The concentrated stream 6′ is then fed back as thefeedstock solution 1.

In an embodiment, the process comprises feeding a high boilingwater-miscible organic solvent to the reaction zone 20, which woulddissolve water-insoluble, nonvolatile byproducts such as humins. In oneembodiment, the high boiling water-miscible organic solvent is added tothe feedstock solution before feeding to the reaction zone 20. Thenonvolatile byproducts can be removed diluting the remaining contents ofthe reboiler in a mixing chamber with water or aqueous feedstocksolution, thereby precipitating water-insoluble byproducts; and removingthe precipitated water-insoluble byproducts, e.g., by filtration orcentrifugation and feeding the precipitate-free solution remaining backto the reaction zone 20.

The water-miscible organic solvent has a boiling point higher than about100° C. at atmospheric pressure. Examples of suitable solvents includewithout limitation: sulfolane, polyethylene glycol, isosorbide dimethylether, isosorbide, propylene carbonate, poly(ethylene glycol) dimethylether, adipic acid, diethylene glycol, 1,3-propanediol, glycerol,gamma-butyrolactone, and gamma-valerolactone.

In one embodiment, the water-miscible organic solvent is sulfolane.

In one embodiment of the invention, the solvent is PEG 4600, PEG 10000,PEG 1000, polyethylene glycol, gamma-valerolactone, gamma-butyrolactone,isosorbide dimethyl ether, propylene carbonate, adipic acid,poly(ethylene glycol)dimethyl ether, isosorbide, Cerenol™ 270(poly(1,3-propanediol), Cerenol™ 1000 ((poly(1,3-propanediol)), ordiethylene glycol.

In one embodiment of the invention disclosed herein, a process isprovided comprising the steps of:

(a) providing reactor comprising a reactive distillation columncomprising an upper, rectifying section; a lower, stripping section; anda reboiler, wherein the stripping section or the reboiler is a reactionzone containing a solid acid catalyst,

(b) continuously feeding an solution comprising C₅ sugar, C₆ sugar or amixture thereof to the column at a location between the rectifyingsection and the stripping section, allowing the solution to flow intothe reaction zone into contact with the solid acid catalyst, therebyforming a reaction mixture, wherein

-   -   (i) the temperature of the reaction mixture is between about        90° C. and about 250° C.    -   (ii) the reaction mixture is held at a pressure between about        atmospheric pressure and about 3.87×10⁶ Pa, and    -   (iii) the sugar solution and catalyst are in contact for a time        sufficient to produce water and furfural;

(c) drawing off a mixture of furfural and water at the top of thecolumn;

(d) collecting water, unreacted sugars, and nonvolatile byproducts inthe reboiler;

(e) removing nonvolatile byproducts from the reboiler; and

(f) removing the water and unreacted sugars from the reboiler forfurther use or disposal.

The combination of high yield and high conversion is desirable for amost efficient and economical process. In the event that a higherselectivity can be obtained at lower conversion, it may be desirable torun at lower conversion, for example 50-80%, and recycle unreactedsugars back to the reaction zone. The process described above producesfurfural from solutions of C5 and/or C6 sugars at both high yield andmedium to high conversion, without production of insoluble char in thereaction vessel. In an embodiment, the furfural yield is in the range of40-95% or 60-95% or 65-85%. In another embodiment, the conversion ofsugar to furfural is in the range of 10-100% or 25-100% or 50-100%. Inan embodiment, the furfural selectivity is in the range of 40-95% or60-95% or 65-85%

Degradation of furfural is minimized by its low residence time incontact with the solid acid catalyst. Higher catalyst lifetime can beachieved because the catalyst is continually washed with the refluxingsolution and not in contact for long periods of time with high-boilingbyproducts like humins, which are known to be deleterious to catalystlifetime. Solid acid catalysts have the advantage of not inducingcorrosion in the reaction vessels and other process equipment ascompared to liquid acid catalysts.

As used herein, where the indefinite article “a” or “an” is used withrespect to a statement or description of the presence of a step in aprocess of this invention, it is to be understood, unless the statementor description explicitly provides to the contrary, that the use of suchindefinite article does not limit the presence of the step in theprocess to one in number.

As used herein, when an amount, concentration, or other value orparameter is given as either a range, preferred range, or a list ofupper preferable values and lower preferable values, this is to beunderstood as specifically disclosing all ranges formed from any pair ofany upper range limit or preferred value and any lower range limit orpreferred value, regardless of whether ranges are separately disclosed.Where a range of numerical values is recited herein, unless otherwisestated, the range is intended to include the endpoints thereof, and allintegers and fractions within the range. It is not intended that thescope of the invention be limited to the specific values recited whendefining a range.

As used herein, the terms “comprises,” “comprising,” “includes,”“including,” “has,” “having,” “contains” or “containing,” or any othervariation thereof, are intended to cover a non-exclusive inclusion. Forexample, a composition, a mixture, process, method, article, orapparatus that comprises a list of elements is not necessarily limitedto only those elements but may include other elements not expresslylisted or inherent to such composition, mixture, process, method,article, or apparatus. Further, unless expressly stated to the contrary,“or” refers to an inclusive or and not to an exclusive or. For example,a condition A or B is satisfied by any one of the following: A is true(or present) and B is false (or not present), A is false (or notpresent) and B is true (or present), and both A and B are true (orpresent).

As used herein, the term “invention” or “present invention is anon-limiting term and is not intended to refer to any single variationof the particular invention but encompasses all possible variationsdescribed in the specification and recited in the claims.

As used herein, the term “about” modifying the quantity of an ingredientor reactant of the invention employed refers to variation in thenumerical quantity that can occur, for example, through typicalmeasuring and liquid handling procedures used for making concentrates oruse solutions in the real world; through inadvertent error in theseprocedures; through differences in the manufacture, source, or purity ofthe ingredients employed to make the compositions or carry out themethods; and the like. The term “about” also encompasses amounts thatdiffer due to different equilibrium conditions for a compositionresulting from a particular initial mixture. Whether or not modified bythe term “about”, the claims include equivalents to the quantities. Theterm “about” may mean within 10% of the reported numerical value,preferably within 5% of the reported numerical value.

EXAMPLES

The methods described herein are illustrated in the following examples.From the above discussion and these examples, one skilled in the art canascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various uses andconditions.

Abbreviations

The meaning of abbreviations is as follows: “cm” means centimeter(s),“g” means gram(s), “h” means hour(s), “HPLC” means high pressure liquidchromatography, “m” means meter(s), “min” means minute(s), “mL” meansmilliliter(s), “mm” means millimeter(s), “MPa” means megapascal(s), “N”means normal, “psi” means pound(s) per square inch, “PTFE” meanspoly(tetrafluoroethylene), “rpm” means revolutions per minute, “wt %”means weight percent(age), “μL” means microliter(s), and “μm” meansmicrometer(s).

Materials Amberlyst® A70 ion exchange resin was manufactured by DowChemical's Rohm and Haas division (Philadelphia, Pa.).

Xylose, sulfolane, and dimethylsulfoxide were obtained fromSigma-Aldrich Corporation (St. Louis, Mo.).

Zeolite CP7146 used in Example 3 was obtained from Zeolyst International(Conshohocken, Pa.)

The following solid acid catalysts were obtained from ZeolystInternational, Conshohocken, Pa., or Conteka B. V. (now Zeolyst,International): Product #: CBV 400, CBV 500, CBV 712, CBV 720, CBV 760,CBV 780, CP 814C, CP 814E, CP 811B-200, CP 811C-300, CBV 3020, CBV 5020,CBV 1502, CBV 2802 (now CBV 28014), CBV 10A, CBV 20A, and CBV 30A. Thesolid acid catalyst S-115 (LA) was obtained from Union CarbideCorporation (now UOP, Des Plaines, Ill.). The solid acid catalystAmberlyst™ 70 was obtained from Dow Chemical Company (Midland, Mich.).Amberlyst™ 70 is a macroreticular polymer based catalyst primarilycomprising sulfonic-acid-functionalized styrene divinylbenzenecopolymers. The solid acid catalyst 13% Nafion® on silica (SiO₂) wasobtained from E. I. du Pont de Nemours and Co. (Wilmington, Del.).Nafion® is a registered trademark of E. I. du Pont de Nemours andCompany for its perfluorinated sulfonic acid polymer products.

Deionized water was used unless otherwise indicated.

Analytical Methods FOR EXAMPLE 1 (PROPHETIC), COMPARATIVE EXAMPLES A ANDB

Furfural and sugar analysis is done by HPLC. Samples were collected andpassed through a 0.2 μm syringe filter prior to analysis. The sampleswere neutralized with calcium carbonate and re-filtered before they wereanalyzed by high pressure liquid chromatography (HPLC). The HPLCinstrument employed was a HP 1100 Series equipped with Agilent 1200Series refractive index (RI) detector and an auto injector (Santa Clara,Calif.). The analytical method was adapted from an NREL procedure(NREL/TP-510-42623). Separation and quantitation of monomeric sugars(glucose, xylose, and arabinose), and furfural (FF) was performed byinjecting the sample (10 μL) on to a Bio-Rad HPX-87P (Bio-Rad, Hercules,Calif.) column maintained at 85° C. Water was used as the eluant, with aflow-rate of 0.6 mL/min. The reaction products in the eluant wereidentified with the RI detector operating at 55° C.

FOR EXAMPLES 2, 3, 4 AND COMPARATIVE EXAMPLE C

Distillates and reaction flask contents were analyzed on a calibratedAminex HPX-87H HPLC column (Bio-Rad Company) using a refractive indexdetector, and the column wash was analyzed via gas chromatographicanalysis using a flame ionization detector and a calibrated 30 mHP-INNOWax GC column (Agilent Technologies).

PROPHETIC EXAMPLE 1 Conversion via Reactive Distillation of SugarSolution to Furfural with Solid Acid Catalyst

A solution of 10 wt % pentose and pentose oligomers with less than 1 wt% hexose and hexose oligomers is fed to a distillation column. The 1inch (2.54 cm) diameter stainless steel distillation column has 5 trayslocated above the feed point and 5 trays with enhanced hold up time pertray loaded with Amberlyst™ A70 sulfonic acid ion exchange resin beads.The column is refluxing water upon startup at a temperature of 180° C.and pressure of 120 psi (0.827 MPa). Feed is begun at 5 grams per minuteabove the stripping/reactive section of the column, and the materialreacts to produce a material comprising furfural, water and high boilersThe reboiler of the distillation column is level controlled with a flowout of 1.33 grams per minute analyzing at, for example, 0.5 wt % inpentose and hexose and oligomers, 8.3 wt % in humins and other highboilers (present primarily as solids), and about 91.2 wt % water.Furfural is not detectable in the reboiler material. The distillate isremoved at a rate of 3.67 grams per minute at the top of the column witha composition of, for example, 7.0 wt % furfural, the remaindercomprising primarily water. The steady state yield to furfural frompentose and pentose oligomers of the process as run in this examplewould be 70.0%.

EXAMPLE 1 Production of Furfural with Solid Acid Catalysts

Zeolites having different frameworks were used as catalysts as indicatedin Table 1, including faujasite (FAU), zeolite beta (BEA), ZSM-5 (MFI),and mordenite (MOR). All zeolites were calcined at 550° C. for 8 h inair prior to use. All of the zeolites are in proton form after calciningexcept for CBV 10A which was in the sodium form. The polymer catalystsAmberlyst™ 70 and 13% Nafion on silica were used as obtained.

The following amounts and variables were the same for all experiments inthis Example: 1) solvent was sulfolane, 2) mass of solvent was 5 g, 3)mass of solid catalyst was 0.075 g (1.5% of the solvent mass), 4)aqueous xylose solution concentration was 5 wt %, 5) xylose solutionaddition rate was 0.4 mL/min, 6) stirring rate was approximately 500rpm, 7) reaction run time was 40 min, 8) average reaction temperaturewas 170° C., 9) oil bath temperature was 250° C., and 10) the internalstandard added for analysis was dimethylsulfoxide.

The conversion of xylose to furfural was carried out in a 10 mLthree-necked round bottomed flask (Chemglass, Inc. Life Sciences CatalogNo. PN CG-1507-03) containing a PTFE-coated stirring bar (VWR CompanyCatalog No. 58949-010), a thermowell, a threaded adapter with cap(Chemglass, Inc. Life Sciences Catalog No. CG-350-01), and a PTFE-linedsilicon septum (National Scientific Catalog No. B7995-15). The flask wasconnected to a vacuum-jacketed Vigreux distillation column (Chemglass,Inc. Life Sciences Catalog No. CG-1242) loaded with 8.0 g of 4 mmdiameter glass beads (Chemglass, Inc. Life Sciences Catalog No.CG-1101-03). The beads were held in place at the bottom of thedistillation column with a piece of 1/16″ diameter thick fluoropolymerfilm that was approximately ¾″ wide by 3″ long which was either wound upinto a coil or folded so that it contained pleats. A 20 mL plasticsyringe with Luer lock tip (Chemglass, Inc. Life Sciences Catalog No. PN309661) was connected to 1/16″ fluoropolymer tubing which was piercedthrough the septum. Addition of the xylose solution from the syringe tothe reaction vessel was controlled with a digital syringe pump. Thereactions were carried out under an atmosphere of nitrogen.

To the reaction flask were added 5 g of solvent and 0.075 g of solidacid catalyst. The syringe on the syringe pump was filled with anaqueous xylose solution which was weighed prior to addition, and thenreweighed after the completion of addition to determine the total amountof xylose solution added to the reaction mixture. After the flask wasloaded, it was attached to the distillation column and one end of the1/16″ diameter fluoropolymer tube was attached to the syringe containingthe aqueous xylose solution and the other end was inserted through theseptum and into the reactor. The flask was lowered into the hot oil tobring the reactor contents to the desired internal temperature andaddition of the xylose solution from the syringe using the syringe pumpwas started. The xylose solution was added at a constant rate and thetemperature of the reaction mixture was maintained as constant aspossible by slight adjustments to the height of the apparatus in the oilbath. At the end of the reaction, the syringe pump was stopped, the tubewas pulled from the reaction flask and the apparatus was raised out ofthe oil bath.

The amount of distillate collected was weighed, a measured amount of theinternal standard (dimethylsulfoxide) was added for analytical purposes,and the solution was then mixed until it was homogeneous (additionalwater was added to dilute the mixture if necessary). The reaction flaskwas removed from the distillation head and was weighed to determine themass of material in the flask. A measured amount of internal standard(dimethylsulfoxide) was added to the reaction flask and it was mixedwell. The contents of the reaction flask were then transferred to a 50mL centrifuge tube. The distillation head was washed with water and thewashes were also used to wash the reboiler. All the washes were combinedin the 50 mL centrifuge tube, and solids were centrifuged to the bottomof the tube using the supernatant for analysis.

The distillate, reaction flask contents, and the washes were thenanalyzed by HPLC on a calibrated Biorad Aminex HPX-87H column using arefractive index detector. An aqueous 0.01 N H₂SO₄ isocratic mobilephase flowing at 0.6 mL/min through a column heated to 65° C. and arefractive index detector heated to 55° C. The detected amounts ofxylose and furfural were recorded. Results for different solid acidcatalysts are presented in Table 1.

TABLE 1 Catalyst Mole Type or Mole Ratio Original Zeolite Ratio (Al/Al +Si) Surface Xylose Selectivity Yield of Catalyst Framework Si/Al in inArea Conversion to Furfural Furfural Run Source Type Catalyst Catalyst(m²/g) (%) (%) (%) 1.1 CP814E BEA 12.5 0.074 680 99 74 73 1.2 CP814C BEA19 0.05 710 97 74 72 1.3 CP811B- BEA 100 0.01 — 94 67 63 200 1.4Amberlyst ™ polymer — — — 93 63 59 70 1.5 CP811C- BEA 150 0.007 620 9162 57 300 1.6 CBV720 FAU 15 0.063 780 89 57 51 1.7 CBV3020 MFI 15 0.063405 92 55 51 1.8 CBV30A MOR 15 0.063 600 91 55 50 1.9 CBV780 FAU 400.024 780 90 55 49 1.10 CBV760 FAU 30 0.032 720 92 52 48 1.11 CBV20A MOR10 0.091 500 91 53 48 1.12 CBV712 FAU 6 0.143 730 87 47 41 1.13 CBV1502MFI 75 0.013 420 86 46 40 1.14 13% polymer — — — 91 44 40 Nafion/ SiO₂1.15 CBV2802 MFI 140 0.007 411 88 43 38 1.16 CBV5020 MFI 25 0.038 425 8742 37 1.17 CBV500 FAU 2.60 0.278 750 84 29 24 1,18 CBV400 FAU 2.55 0.282730 83 25 20

The catalysts that gave the highest yields in these experiments were thebeta zeolites, particularly catalysts derived from calcinations ofCP814C and CP814E. Amberlyst™ 70 also gave high yields and conversion.

EXAMPLE 2 Dehydration of Xylose to Furfural via Reactive Distillationwith Solid Acid Catalyst

The reactive distillation unit used here consisted of a jacketed glasstube reactor. The glass reactor, present inside the outer jacket, had alength of 8.5 inch (21.6 cm) and an outer diameter of 1.38″ (3.5 cm).The glass reactor was filled with about 10 gm of beta-zeolite catalyst(granules). The catalyst was provided by Zeolyst International (product#CP814E, lot #2200-42, SiO₂:Al₂O₃ mol ratio 25:1, Si: Al ratio 12.5:1,surface area 680 m²/g) in a powder form. The powder was calcined in airat 550° C. for about 8 h. The calcined powder was then charged instainless steel die and pressed at 1.82×10⁵ kPa using a Preco hydraulicpress. The resulting slugs (25 mm diameter×˜25 mm thick) were crushedand sieved to produce granules of −12/+14 mesh (1.40 mm−1.70 mm). Thesebeta-zeolite granules were used as catalyst in this study.

The catalyst bed was positioned in the middle of the glass reactor andthe rest of the reactor was packed with glass beads (Chemglass Inc.Catalog No. CG-1101-01) of 2 mm diameter, placed above (strippingsection) and below the reactor. A stainless steel mesh was placed belowthe glass beads (in the bottom portion of the reactor) to support thecatalyst bed and glass beads. A thermocouple was used to monitor thecatalyst bed temperature and was placed inside a thermowell located inmiddle of the glass reactor.

The inner glass reactor was surrounded by an outer jacket (Outerdiameter: 5.7 cm) through which an oil (Lauda Brinkmann LZB 222,THERM240) was circulated continuously in order to maintain the reactortemperature at a desired value. A high temperature oil bath (NeslabExacal EX-250HT) was used to control the oil temperature, and the flowrate of the oil through the outer jacket of the reactor. The oil bathtemperature was kept at 195-200° C. so that an average temperature ofabout 175° C. was maintained in the catalyst bed (placed inside theglass reactor).

A distillation head including a condenser was attached to the top of thereactor, where a temperature of 15° C. was maintained constant with acontinuous circulation of a coolant mixture containing 50 wt. % ethyleneglycol (VWR, BDH 2033) and 50 wt. % water. A circulation bath (LaudaEcoline Staredition RE112) was used for this purpose.

The reactor was connected to a continuous flow system capable ofprecisely controlled liquid feed delivered by an HPLC Pump (Lab AllianceSeries I). In this particular study of catalyst activity and stabilityfor a beta-zeolite catalyst sample, a feed consisting of 5 wt. % xylose(Sigma Aldrich, X1500) in a mixture that contained 15 wt % water and 80wt. % high boiling solvent, sulfolane (Sigma Aldrich, T22209), wasloaded to the HPLC pump. The feed rate (to the reactor) was maintainedconstant at 0.75 ml/min. A glass container filled with the above feedsolution was kept on a balance to continuously monitor the amount offeed introduced to the reactor. The feed was introduced above thecatalyst bed at the specified feed rate. The stripping section(containing glass beads) also aided in uniformly distributing the liquidfeed to the catalyst bed. The feed mixture reacted on the catalyst bedto form furfural, which was the desired product of this reaction alongwith some high boilers and water. Water and furfural being low boilers,formed vapors and travelled to the distillation head containing thecondenser. The vapors were then condensed and were collected in a glassflask (250 ml, Chemglass Inc., Catalog No. CG-1559-10) surrounded by anice bath (for the purpose of providing a low temperature atmosphere forfurther cooling the vapors). One of the necks of this flask was sealedwith a rubber septum. A 10 ml plastic syringe with Luer lock tip (BD,REF 309604) was connected to a needle which was pierced through theseptum. This syringe was used to collect the distillate sample atregular intervals. The reaction was carried out under atmosphericpressure.

The reactive distillation unit was also equipped with a reflux valve,which was closed (reflux ratio=0), in order to avoid the reactionbetween furfural (with itself, forming oligomers of furfural) and xylosefurther resulting in the formation of high boilers, commonly known ashumins. The unreacted feed (containing xylose and sulfolane) along withhigh boilers (humins) formed during the reaction was collected in thereboiler located below the reactor. The reboiler was a 3-neckedround-bottom glass flask (250 ml, Chemglass Inc., Catalog No.CG-1530-01). A 20 ml plastic syringe with Luer lock tip (BD 20 mlsyringe REF 309661) was connected to a needle, which was pierced througha rubber septum used to seal one of the necks of the round bottom flask.This syringe was used to collect the reboiler sample at regularintervals. Another neck of the round bottom flask was sealed with arubber septum and a ⅛″ Teflon tubing (Chemglass Inc., Catalog No.CG-1037-10) was pierced through the septum into the reboiler. Thistubing was used to introduce water to the reboiler at a constant rate of0.50 ml/min maintained with the help of a digital syringe pump (KDScientific, Model No. KDS KEGATO 270, Catalog No. 78-8270). The reboilerwas kept heated at a temperature of 160° C. With this sufficient hightemperature and a continuous input of water in the reboiler, there was asteady formation of steam which traveled up through the catalyst bed andfurther helped to effectively remove furfural (by forming an azeotrope)from the reaction zone. Thus the steam stripping brings additionaladvantage of effective furfural separation from the reaction zone. N₂was also introduced in one of the necks of the reboiler, for furtherremoval of furfural from the reaction zone.

The samples (both distillate and reboiler) were collected in glass vialsand weighed. The reboiler samples collected during the reactivedistillation run were analyzed by HPLC on a calibrated Biorad AminexHPX-87H column using a refractive index detector. An aqueous 0.01 NH2SO4 isocratic mobile phase flowing at 0.6 ml/min through a columnheated to 65° C. and a refractive index detector heated to 55° C. Ameasured amount of the internal standard (dimethylsulfoxide) was addedfor analytical purposes, and the solution was then mixed until it washomogeneous. The detected amounts of xylose and furfural were recorded.The distillate samples were analyzed by an Agilent 6890GC equipped witha 30 meter DB-1 capillary column (J&W 125-1032). 5 microliters ofsolution was injected into an injector port set to 175° C. with a splitratio of 5:1, a total helium flow of 55.2 ml/min, a split flow of 44.4ml/min and a head pressure of 6.25 psi. The oven temperature was held at50° C. for 2 min and then it was increased to 110° C. at 10° C./minfollowed by a second increase to 240° C. at 20° C./min. A flameionization detector set at 250° C. was used to detect signal. A measuredamount of the internal standard (1-pentanol) was added for the GCanalysis. The detected amounts of furfural were recorded. Resultsobtained during the dehydration of xylose to furfural using beta-zeolitecatalyst have been presented in Table 2.

Table 2 below shows the result of a 3-day run (140 min on day 1, 390 minon day 2 and 150 min on day 3) carried out for about 12 hours (underidentical conditions of temperature, flow rate, etc.). The data arereported for the steady state conditions achieved in the reactor. Asseen in the table 2, the beta-zeolite catalyst resulted in xyloseconversion of greater than 95%. The furfural yields (and hence theselectivity towards furfural) were nearly steady over the entire run.

TABLE 2 Dehydration of Xylose to Furfural via Reactive DistillationUsing beta-Zeolite Catalyst and a Feed Containing 5 wt. % Xylose, 15 wt.% Water and 80 wt. % Sulfolane. Time Xylose Furfural (min) ConversionFurfural Yield Selectivity 80 99.5% 66.8% 67.2% 110 99.3% 72.7% 73.2%140 98.9% 69.2% 70.0% 290 96.9% 69.4% 71.7% 350 95.5% 69.7% 73.0% 41094.5% 69.5% 73.6% 470 96.4% 65.0% 67.5% 680 97.8% 70.3% 71.8%

EXAMPLE 3 Xylose Reactive Distillation Using H-Mordenite Catalyst

Above experimental set up (in Example 3) was then used to study thedehydration of xylose to furfural reaction using H-mordenite catalyst.The H-mordenite catalyst used here was provided by Zeolyst International(Product #CBV21A, lot #2200-77, SiO₂/Al₂O₃ mol ratio 20:1, Si:Al ratio10:1, surface area 500 m²/g) in a powder form. The powder catalyst wasthen calcined and converted into granules using a similar techniquedescribed earlier for the beta-zeolite catalyst. The rest of theexperimental conditions were the same as used earlier for thebeta-zeolite catalyst (feed composition: 5 wt. % xylose, 15 wt. % water,80 wt. % sulfolane; feed flow rate=0.75 ml/min; water folw rate in thepot=0.50 ml/min; pot temperature=160 ° C., etc.) The reactor temperaturewas maintained in the range of 175-180° C. Table 3 summarizes theresults for the H-mordenite catalyst.

TABLE 3 Dehydration of Xylose to Furfural via Reactive DistillationUsing H-mordenite Catalyst and a Feed Containing 5 wt. % Xylose, 15 wt.% Water and 80 wt. % Sulfolane. Time (min) Xylose Conversion FurfuralYield Furfural Selectivity 70 99.0% 63.4% 64.0% 105 98.8% 72.7% 73.6%165 97.7% 74.5% 76.3% 225 97.0% 74.6% 76.9% 290 98.5% 82.4% 83.6%

Example 3 gives an example of production of furfural with reactivedistillation utilizing a high boiling solvent resulting in a higheryield than seen in the Comparative Examples. Example 4 shows an evenhigher yield of furfural than seen in Example 3.

Comparative Example A described below gives a comparison run using afixed bed reactor with an acidic ion exchange resin and no high boilingsolvent with a much poorer resulting yield and selectivity to furfural.Comparative Example B described below gives a comparison with a fixedbed reactor using a beta Zeolite catalyst and a high boiling solvent,similar to that used in example 3, with a worse result for yield.

COMPARATIVE EXAMPLE A Lab-scale Continuous Process: Fixed Bed Reactorwith Aqueous Xylose Feed and Strongly Acidic Ion Exchange Resin Catalyst

A 5 inch (12.7 cm) long, ½″ (1.27 cm) outer diameter of 316 stainlesssteel tubing (Swagelock Corporation) was used as a fixed bed reactor.The catalyst bed was supported by a ⅜″ (0.952 cm) steel tube at thebottom of the upflow arrangement, with a stainless mesh supported bythis tube as bed support for the catalyst. The reactor tube was loadedwith 3 cm³ of Amberlyst™ A70 ion exchange resin. The reactor was thenconnected to a continuous flow system capable of precisely controlledliquid feed delivered by an ISCO D-500 Syringe Pump (Teledyne ISCO,Lincoln Nebr., USA). The reactor was installed within a tube furnacewhich allowed temperature control of the catalyst bed as read by aninternal 1/16″ (15.9 mm) stainless steel thermocouple. The flow exitingthe reactor was then pressure controlled by a Swagelock backpressureregulator capable of up to 1000 psig (6.89 MPa-g) at the chosen liquidflows. The product from the regulator was then collected in sample vialsfor analysis by HPLC.

In the study of catalyst activity and lifetime for Amberlyst™ A70, thefeed was 4 wt % xylose in water, loaded to the ISCO pump. The solutionwas fed through the reactor which was loaded as described previouslywith 3 cm³ of Amberlyst™ A70 acidic ion exchange resin. The reactor wascontrolled at 160° C. via a tube furnace and the pressure was controlledat 200 psig (1.38 MPa-g) by a backpressure regulator. Table 4 belowshows the result of a continuous run where the flowrate was changed tostudy the effect of space velocity on the xylose conversion and furfuralyield in an upflow fixed bed system. Also shown in the table is acalculated first order rate constant (k) for xylose conversion whichpermits comparison of catalyst activity as a function of time. As seenin the table, the activity is low from the start of the run, with adramatic decrease over the course of the experiment. The buildup ofhumins is believed to be the primary cause of catalyst activity loss.

TABLE 4 Furfural Production in a Fixed Bed Solid Acid Reactor with TimeSpace k % Time on velocity Xylose Furfural Furfural (1/ Initial Stream(h) 1/h Conversion Yield Selectivity min) Activity 2.5 16 40.4% 11.6%28.9% 0.138 100.0% 2.7 16 40.6% 11.3% 27.8% 0.139 100.6% 3.8 32 20.2%7.5% 37.2% 0.120 87.2% 3.9 32 23.1% 7.1% 30.9% 0.140 101.6% 22.5 4 30.4%8.8% 29.0% 0.024 17.6% 23.0 4 31.5% 8.5% 27.1% 0.025 18.3% 27.1 8 14.7%4.8% 32.5% 0.021 15.4% 27.3 8 11.6% 5.0% 43.0% 0.016 11.9% 29.3 16 9.4%2.8% 29.7% 0.026 19.2% 29.5 16 9.6% 2.7% 27.8% 0.027 19.5% 35.2 2 46.6%12.4% 26.6% 0.021 15.2% 35.4 2 46.4% 12.1% 26.0% 0.021 15.1%

COMPARATIVE EXAMPLE B Lab-scale Continuous Process: Fixed Bed Reactorwith Sulfolane solvent. And Zeolite Beta Catalyst

The apparatus of Example 2 was used with a high boiling water-misciblesolvent. Sulfolane, in addition to being high boiling, is an excellentsolvent for biomass and humins (by-products from furfural synthesis). Itis hoped that use of such a solvent will increase the lifetime of asolid acid catalyst used for production of furfural from xylose.

In the study of catalyst activity and lifetime for a Zeolite Betasample, the feed was 4 wt % xylose in a mixture that contained 10 wt %water and 86 wt % sulfolane, loaded to the ISCO pump. CP 7146 (ZeolystInternational) is an extruded form of ammonium-beta (CP 814E (Zeolyst),Si/Al=12.5). The sample was calcined by heating in air to 525 deg C. ata rate of 10 deg C./min, then 2 deg C./min to 540 deg C. and finally 1deg C./min to 550 deg C. where the sample was held for 8 hours. 1.4935grams of CP 7146 was loaded to the tubular reactor of ComparativeExample A. The reactor was controlled at 160° C. via a tube furnace andthe pressure was controlled at 200 psig (1.38 MPa-g) by a backpressureregulator. Table 5 below shows the result of a continuous run where theflowrate was changed to study the effect of space velocity on the xyloseconversion and furfural yield in an upflow fixed bed system. Also shownin the table is a calculated first order rate constant (k) for xyloseconversion which permits comparison of catalyst activity as a functionof time. As seen in the table, the activity is much better in sulfolanesolvent than in an aqueous system such as Comparative Example A. Thereis however a dramatic decrease over the course of the experiment as seenin Comparative Example A. The buildup of humins is believed to be theprimary cause of catalyst activity loss. The use of sulfolane solvent,which solubilizes humins, apparently does not prevent the deactivationof the catalyst.

TABLE 5 Furfural Production in a Fixed Bed Solid Acid Reactor with Time,Sulfolane Solvent with Zeolite Beta Catalyst Time on Space % Streamvelocity Xylose Furfural Furfural k Initial (h) 1/h Conversion YieldSelectivity (1/min) Activity 4.0 8 94.5% 44.0% 46.5% 0.386 100.0% 4.2 894.5% 43.9% 46.5% 0.386 100.1% 21.0 2 99.5% 50.5% 50.7% 0.174 45.1% 21.32 99.5% 50.5% 50.8% 0.174 45.1% 25.7 8 66.3% 30.5% 46.0% 0.145 37.6%25.9 8 64.9% 30.1% 46.3% 0.140 36.2% 29.5 8 91.4% 46.7% 51.1% 0.32885.0% 29.8 8 91.0% 46.3% 50.9% 0.321 83.3% 33.8 8 42.4% 8.4% 19.8% 0.07419.1% 34.0 8 42.2% 8.4% 19.8% 0.073 19.0% 36.4 16 31.7% 2.5% 7.8% 0.10226.3% 36.6 16 32.0% 2.6% 8.2% 0.103 26.7% 50.9 2 71.6% 22.1% 30.9% 0.04210.9% 51.5 2 71.9% 22.3% 31.0% 0.042 11.0%

COMPARATIVE EXAMPLE C

Using an analogous procedure as described in Example 2, the materialsderived from S-115 (LA) and CBV 10A after calcination were tested ascatalysts for production of furfural. The results are presented in Table5.

TABLE 5 Mole Selectivity ratio Mole ratio Surface Xylose to Yield ofCatalyst Catalyst Si/Al in (Al/Al + Si) Area Conversion FurfuralFurfural Run Name Type Catalyst in Catalyst (m²/g) (%) (%) (%) C.a S-115MFI 400 0.002 411 77 1 0 (LA) C.b CBV MOR 5 0.167 425 15 0 0 10A

The catalysts derived from S-115 (LA), a zeolite with very low aluminumcontent, and CBV 10A, a zeolite with sodium cations and few Bronstedacid sites, showed 0% yield of furfural in Run A and Run B. Thisdemonstrated that zeolites with a low number of Bronsted acid sites, orlow aluminum content (very high Si/Al ratio, greater than or equal to400) were not good catalysts for furfural production from C₅ and/or C₆sugars.

1. A process comprising: (a) providing a reactive distillation columncomprising a top, a bottom, a reaction zone in between the top and thebottom, and a solid acid catalyst disposed in the reaction zone; (b)bringing a feedstock solution into contact with the solid acid catalystin the presence of a water-miscible organic solvent for a residence timesufficient to produce a mixture of water and furfural, wherein thefeedstock solution comprises C₅ sugar, C₆ sugar or a mixture thereof,and the reaction zone is at a temperature in the range of 90-250° C. anda pressure in the range of 0.1-3.87 MPa; (c) removing the mixture ofwater and furfural from the top of the reactive distillation column; and(d) collecting water, unreacted sugars and nonvolatile byproducts fromthe bottom of the reactive distillation column in a reboiler, whereinthe water-miscible organic solvent is polyethylene glycol, isosorbidedimethyl ether, isosorbide, propylene carbonate, polyethylene glycoldimethyl ether, adipic acid, diethylene glycol, 1,3-propane diol,glycerol, gamma-butyrolactone or gamma-valerolactone.
 2. The processaccording to claim 1, wherein the acid catalyst comprises aheterogeneous heteropolyacid, a salt of a heterogeneous heteropolyacid,a natural or synthetic clay mineral, a cation exchange resin, a metaloxide, a mixed metal oxide, a metal sulfide, a metal sulfate, a metalsulfonate, sulfated titania, sulfated zirconia, a metal nitrate, a metalphosphate, a metal phosphonate, a metal molybdate, a metal tungstate, ametal borate, or a combination of any of these.
 3. The process accordingto claim 2, wherein the acid catalyst comprises a cation exchange resinthat is a sulfonic-acid-functionalized polymer.
 4. The process accordingto claim 2, wherein the acid catalyst comprises a clay mineral that is azeolite.
 5. The process according to claim 4, wherein the acid catalystis a medium or large pore, acidic, hydrophobic zeolite.
 6. The processaccording to claim 5, wherein the zeolite comprises ZSM-5, faujasite,beta zeolite, Y zeolite, mordenite, or a combination of any of these. 7.The process according to claim 1 further comprising: e. removing waterand unreacted sugars from the water, unreacted sugars and nonvolatilebyproducts of step (d); and f. concentrating by evaporation at least aportion of the water and unreacted sugars and using it as feedstocksolution in step (b).
 8. The process according to claim 1 furthercomprising separating the furfural from the removed water and furfuralof step (c).
 9. The process according to claim 1 wherein the combinedconcentration of C₅ sugar and/or C₆ sugar in the feedstock solution isin the range of 1-99 weight percent based on the total weight of thefeedstock solution.
 10. The process according to claim 9 wherein thecombined concentration of C₅ sugar and/or C₆ sugar in the feedstocksolution is in the range of 5-35 weight percent based on the totalweight of the feedstock solution.
 11. The process according to claim 1wherein the feedstock solution comprises xylose, glucose, or a mixturethereof.
 12. (canceled)
 13. The process according to claim 1, furthercomprising a steam-stripping step, comprising feeding water or steam tothe reaction zone from the bottom of the reactive distillation column.14. The process of claim 1 further comprising the steps of: h) dilutingat least a portion of the contents of the reboiler with water or withthe feedstock solution, thereby precipitating water-insolublebyproducts; i) removing the byproducts precipitated in step h); and j)feeding the precipitate-free solution remaining after step i) back tothe reaction zone.
 15. A process comprising the steps of: (a) providinga reactor comprising a reactive distillation column comprising an upper,rectifying section; a lower, stripping section; and a reboiler, whereinthe stripping section or the reboiler is a reaction zone containing asolid acid catalyst, (b) continuously feeding a feedstock solutioncomprising C₅ sugar, C₆ sugar or a mixture thereof to the column at alocation between the rectifying section and the stripping section,allowing the solution to flow into the reaction zone into contact withthe solid acid catalyst in the presence of a water-miscible organicsolvent, thereby forming a reaction mixture, wherein (i) thewater-miscible organic solvent forms a monophasic solution with thewater in the reaction zone and the temperature of the reaction mixtureis between about 90° C. and about 250° C. (ii) the reaction mixture isheld at a pressure between atmospheric pressure and 3.87 MPa, and (iii)the sugar solution and catalyst are in contact for a time sufficient toproduce water and furfural (c) drawing off a mixture of furfural andwater at the top of the column (d) collecting water, unreacted sugars,and nonvolatile byproducts dissolved in the water-miscible organicsolvent in the reboiler; (e) removing nonvolatile byproducts from thereboiler; and (f) removing the water and unreacted sugars from thereboiler for further use or disposal, wherein the water-miscible organicsolvent is polyethylene glycol, isosorbide dimethyl ether, isosorbide,propylene carbonate, polyethylene glycol dimethyl ether, adipic acid,diethylene glycol, 1,3-propane diol, glycerol, gamma-butyrolactone orgamma-valerolactone.